Catalytic processes for the conversion of hydrocarbons are well known and extensively used. In many of these processes, the catalyst consists of particles that are transported between two or more catalyst-containing vessels. The reason why the catalyst is transported varies depending on the process. For example, the catalyst may be transported from one reaction vessel into another reaction vessel in order to take advantage of different reaction conditions in the two vessels in order to improve product yields. In another example, the catalyst may first be transported from a reaction vessel into a regeneration vessel in order to rejuvenate the catalyst, and after rejuvenation, the catalyst may be transported back to the reaction vessel.
The vessels between which the catalyst is transported are not necessarily adjacent, and indeed it is common that the outlet of the source vessel, that is the vessel from which the catalyst is transported, may be a significant distance horizontally and vertically from the inlet of the destination vessel, that is the vessel to which the catalyst is transported. An inexpensive and common method of transferring catalyst over significant vertical and horizontal distances is by pneumatic conveying through a conduit. Pneumatic conveying is well known to those skilled in the art of transporting particles. Pneumatic conveying is described at pages 5-46 to 5-48 in Perry's Chemical Engineers' Handbook, Sixth Edition, ed. by Don W. Green, McGraw-Hill ed., McGraw-Hill Book Company, New York, 1984.
One of the problems associated with pneumatic conveying is that the pressure difference across the conduit between the source and destination vessels varies depending on the transport rate of catalyst through the conduit. For example, the pressure difference across the conduit when gas is flowing at its design rate and no catalyst is flowing may be only 1-5 in. H.sub.2 O, but the pressure difference when gas and catalyst are both flowing at their design rates may be 150-250 in. H.sub.2 O. Without a means for controlling this increase in pressure difference, either the pressure in the source vessel will rise or the pressure in the destination vessel will fall. In those processes where catalyst is entering the source vessel by gravity flow at the same time that catalyst is being transported out of the source vessel through the conduit, a surge of 150-250 in. H.sub.2 O in the pressure of the source vessel can stop or even reverse the flow of catalyst into the source vessel. This situation is unacceptable in those processes in which it is important that the flow of catalyst into the source vessel be continuous or that the pressure of the source vessel be kept from changing rapidly.
The pressure difference across a conduit has been used as an indicator in methods of controlling the rate of pneumatic transport of particles through the conduit. Typically, these methods have comprised providing a controller with a desired value of the pressure difference, measuring with a device the actual value of the pressure difference, comparing the desired and actual values, and finally changing the rate of transport until the actual and desired values of the pressure difference are substantially equal.
Control methods like the ones just described suffer from large and rapid fluctuations in the pressures in the source zone, the destination zone, or both. Changes from one value of the desired pressure difference to another value of the desired pressure difference are made instantaneously and in one step. Changes in the rate of transport cause fluctuations that necessitate the use of larger and/or extra vessels and higher rates for making up and venting gases from the process in order to attempt to control the pressure fluctuations.